An alloy within the confines of U.S. Pat. No. 4,358,511 and sold commercially is generally heat treated after solutioning and cold working by aging the alloy at about 732-733.degree. C. for 1 to about 24 hours, furnace cooling the cold worked and aged alloy to about 621-622.degree. C., holding at that temperature for about 8 hours and then cooling in air. In so far as we are aware this procedure results in alloy objects, structures and the like which are adapted to be employed under high stress in sour gas oil well environments without danger of stress corrosion cracking. The solution treated cold worked and aged alloy generally exhibits a 0.2% offset Yield Strength at room temperature of at least 689 MPa.
A different situation prevails if the alloy is not cold worked after solution treatment. Slow strain rate tensile tests conducted at a temperature of 204.degree. C. in an aqueous chloride medium slightly acidified with acetic acid and containing hydrogen sulfide have shown that non-cold worked specimens of the commercial alloy aged at 732.degree. C. to greater than 590 MPa e.g., greater than 689 MPa 0.2% offset yield strength at room temperature are sensitive to stress corrosion cracking. This laboratory observation duplicates practical experience of stress corrosion cracking of valve bodies made of the non-cold worked commercial alloy heat treated as described above.
The problem is to provide large section alloy bodies, e.g., valve bodies, tube hangers, drill collars, various other items of oil well tooling, etc., which are not cold worked after solution treatment, which are aged to a 0.2% offset Yield Strength at room temperature of at least 517 MPa and which are resistant to stress corrosion cracking. Needless to say, other mechanical characteristics of engineering significance of the commercial alloy such as Ultimate Tensile Strength, ductility, impact resistance, etc. should not be detrimentally affected by whatever means are employed to provide a solution to the problem. Specifically, the alloy body should be free of detrimental phases such as sigma phase.